Control system for an N-methyl-2-pyrrolidone refining unit receiving light sweet charge oil

ABSTRACT

A refining unit treats light sweet charge oil with an N-methyl-2-pyrrolidone solvent, hereafter referred to as MP, in a refining tower to yield raffinate and extract mix. The MP is recovered from the raffinate and from the extract mix and returned to the refining tower. A system controlling the refining unit includes a gravity analyzer, a sulfur analyzer, and viscosity analyzers; all analyzing the light sweet charge oil and providing corresponding signals, sensors sense the flow rates of the charge oil and the MP flowing into the refining tower and the temperature of the extract mix and provide corresponding signals. One of the flow rates of the light sweet charge oil and the MP flow rates is controlled in accordance with the signals from all the analyzers and all the sensors, while the other flow rate of the light sweet charge oil and the MP flow rates is constant.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to control systems and methods in general and, more particularly, to control systems and methods for oil refining units.

SUMMARY OF THE INVENTION

A refining unit treats light sweet charge oil with an N-methyl-2-pyrrolidone solvent, hereafter referred to as MP, in a refining tower to yield raffinate and extract mix. The MP is recovered from the raffinate and from the extract mix and returned to the refining tower. A system controlling the refining unit includes a gravity analyzer, a sulfur analyzer, and viscosity analyzers. The analyzers sample the light sweet charge oil and provide corresponding signals. Sensors sense the flow rates of the charge oil and the MP flowing into the refining tower and the temperature of the extract-mix and provide corresponding signals. The flow rate of the light sweet charge oil or the MP is controlled in accordance with the signals provided by all the sensors, the refractometer and the analyzers while the other flow rate of the light sweet charge oil or the MP is constant.

The objects and advantages of the invention will appear more fully hereinafter from a consideration of the detailed description which follows, taken together with the accompanying drawings wherein one embodiment of the invention is illustrated by way of example. It is to be expressly understood, however, that the drawings are for illustration purposes only and are not to be construed as defining the limits of the invention.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a refining unit in partial schematic form and a control system, constructed in accordance with the present invention, in simple block diagram form.

FIG. 2 is a detailed block diagram of the control means shown in FIG. 1.

FIGS. 3 through 13 are detailed block diagrams of the H computer, the K signal means, the H signal means, the KV computer, the VI signal means, the SUS computer, the SUS₂₁₀ computer, the VI_(DWC).sbsb.O computer, the VI_(DWC).sbsb.P computer, the ΔRI computer and the J computer, respectively, shown in FIG. 2.

DESCRIPTION OF THE INVENTION

An extractor 1 in a refining unit is receiving light sweet charge oil by way of a line 4 and N-methyl-2-pyrrolidone solvent, hereafter referred to as MP, by way of a line 7 and providing raffinate to recovery by way of a line 10, and an extract mix to recovery by way of a line 14. The raffinate is subsequently processed to yield refined oil.

Light sweet charge oil is a charge oil having a sulfur content equal to or less than a predetermined sulfur content and having a kinematic viscosity, corrected to a predetermined temperature, less than a predetermined kinematic viscosity. Preferably, the predetermined sulfur content is 1.0%, the predetermined temperature is 210° F., and the predetermined kinematic viscosity is 7.0. The temperature in extractor 1 is controlled by cooling water passing through a line 16. A gravity analyzer 20, viscosity analyzers 23 and 24, and a sulfur analyzer 28 sample the charge oil in line 4 and provide signals API, KV₂₁₀, KV₁₅₀ and S, respectively, corresponding to the API gravity, the kinematic viscosities at 210° F. and 150° F., and sulfur content, respectively, of the light sweet charge oil.

A flow transmitter 30 in line 4 provides a signal CHG corresponding to the flow rate of the charge oil in line 4. Another flow transmitter 33 in line 7 provides a signal SOLV corresponding to the MP flow rate. A temperature sensor 38, sensing the temperature of the extract mix leaving extractor 1, provides a signal T corresponding to the sensed temperature. All signals hereinbefore mentioned are provided to control means 40.

Control means 40 provides signal C to a flow recorder controller 43. Recorder controller 43 receives signals CHG and C and provides a signal to a valve 48 to control the flow rate of the charge oil in line 4 in accordance with signals CHG and C so that the charge oil assumes a desired flow rate. Signal T is also provided to temperature controller 50. Temperature controller 50 provides a signal to a valve 51 to control the amount of cooling water entering extractor 1 and hence the temperature of the extract-mix in accordance with its set point position and signal T.

The following equations are used in practicing the present invention for light sweet charge oil:

    H.sub.210 =1n1n(KV.sub.210 +C.sub.1)                       (1)

where H₂₁₀ is a viscosity H value for 210° F., KV₂₁₀ is the kinematic viscosity of the charge oil at 210° F. and C₁ is a constant having a preferred value of 0.7.

    H.sub.150 =1n1n(KV.sub.150 +C.sub.1)                       (2)

where H₁₅₀ is a viscosity H value for 150° F., and KV₁₅₀ is the kinematic viscosity of the charge oil at 150° F.

    K.sub.150 =[C.sub.2 -1n(T.sub.150 +C.sub.3 ]/C.sub.4       (3)

where K₁₅₀ is a constant needed for estimation of the kinematic viscosity at 100° F., T₁₅₀ is 150, and C₂ through C₄ are constants having preferred values of 6.5073, 460 and 0.17937, respectively.

    H.sub.100 =H.sub.210 +(H.sub.150 -H.sub.210)/K.sub.150     (4)

where H₁₀₀ is a viscosity H value for 100° F.

    KV.sub.100 =exp[exp(H.sub.100)]-C.sub.1                    (5)

where KV₁₀₀ is the kinematic viscosity of the charge oil at 100° F.

    ______________________________________                                         6. SUS =  C.sub.5 (KV.sub.210) + [C.sub.6 + C.sub.7 (KV.sub.210)]/[C.sub.8                + C.sub.9                                                                     (KV.sub.210) + C.sub.10 (KV.sub.210).sup.2 + C.sub.11 (KV.sub.21               0).sup.3 ](C.sub.12)                                                 ______________________________________                                    

where SUS is the viscosity in Saybolt Universal Seconds and C₅ through C₁₂ are constants having preferred values of 4.6324, 1.0, 0.03264, 3930.2, 262.7, 23.97, 1.646 and 10⁻⁵, respectively.

    SUS.sub.210 =[C.sub.13 +C.sub.14 (C.sub.15 -C.sub.16)]SUS  (7)

where SUS₂₁₀ is the viscosity in Saybolt Universal Seconds at 210° F. and C₁₃ through C₁₆ are constants having preferred values of 1.0, 0.000061, 210 and 100, respectively.

    ______________________________________                                          8. VI.sub.DWCO.sbsb.O =                                                                 --C.sub.17 --C.sub.18 (S) + C.sub.19 (KV.sub.210).sup.2 +                      C.sub.20 (VI).sup.2,                                                           +C.sub.21 (S).sup.2 + C.sub.22 (API)(KV.sub.210)--C.sub.23                     (KV.sub.210)                                                                   (VI) + C.sub.24 (VI)(S)                                              ______________________________________                                    

where VI_(DWC).sbsb.O is the viscosity index of the dewaxed charge oil having a pour point of 0° F., and C₁₇ through C₂₄ are constants having preferred values of 18.067, 51.155, 1.0108, 0.0084733, 2.2188, 1.0299, 0.34233 and 0.67215, respectively.

    ______________________________________                                          9. VI.sub.DWC.sbsb.P =                                                                    VI.sub.DWC.sbsb.O + (Pour)[C.sub.25-C26 lnSUS.sub.210                          +C.sub.27 (lnSUS.sub.210).sup.2 ]                                  ______________________________________                                    

where VI_(DWC).sbsb.P and Pour are the viscosity index of the dewaxed charge at a predetermined pour point temperature and the Pour Point of the dewaxed product, respectively, and C₂₅ through C₂₇ are constants having preferred values of 2.856, 1.18 and 0.126, respectively.

    ΔVI=VI.sub.RO -VI.sub.DWC.sbsb.O =VI.sub.RP -VI.sub.DWC.sbsb.P (10)

where VI_(RO) and VI_(RP) are the VI of the refined oil at 0° F., and the predetermined temperature, respectively.

    ______________________________________                                         11. ΔRI =                                                                        [C.sub.28 + C.sub.29 (KV.sub.210) - C.sub.30 (S).sup.2 + C.sub.31              (ΔVI) (API)                                                              -C.sub.32 (API).sup.2 + C.sub.33 (API) (KV.sub.210) + C.sub.34                 (VI).sup.2                                                                     -C.sub.35 (KV.sub.210) (VI) + C.sub.36 (VI) (S) + C.sub.37                     (ΔVI)                                                                     (KV.sub.210)]C.sub.38                                                 ______________________________________                                    

where ΔRI is the change in refractive index between the light sweet charge oil and the raffinate and C₂₈ through C₃₈ are constants having preferred values of 99.848, 41.457, 32.735, 0.116, 0.37573, 23635, 0.03488, 1,3274, 1.2068, 0.25432 and 10⁻⁴, respectively.

    ______________________________________                                         12. J =                                                                               -C.sub.39 + C.sub.40 (ΔRI) + C.sub.41 (S).sup.2 - C.sub.42               (KV.sub.210) (T)                                                               +C.sub.43 (VI) - C.sub.44 (S) + C.sub.45 (ΔRI) (ΔVI)               -C.sub.46 (ΔVI) (T)                                               ______________________________________                                    

where J is the methyl-2-pyrrolidone dosage and C₃₉ through C₄₆ are constants having preferred values of 31.022, 12315, 558.75, 0.08962, 2.9954, 860.35, 496.1 and 0.062708, respectively.

    C=(SOLV)(100)/J                                            (13)

where C is the new charge oil flow rate.

Referring now to FIG. 2, signal KV₂₁₀ is provided to an H computer 50 in control means 40, while signal KV₁₅₀ is applied to an H computer 50A. It should be noted that elements having a number and a letter suffix are similar in construction and operation to those elements having the same numeric designation without a suffix. All elements in FIG. 2, except elements whose operation is obvious, will be disclosed in detail hereinafter. Computers 50 and 50A provide signals E₁ and E₂ corresponding to H₂₁₀ and H₁₅₀, respectively, in equations 1 and 2, respectively, to H signal means 53. K signal means 55 provides a signal E₃ corresponding to the term K₁₅₀ in equation 3 to H signal means 53. H signal means 53 provides a signal E₄ corresponding to the term H₁₀₀ in equation 4 to a KV computer 60 which provides a signal E₅ corresponding to the term KV₁₀₀ in accordance with signal E₄ and equation 5 as hereinafter explained.

Signals E₅ and KV₂₁₀ are applied to VI signal means 63 which provides a signal E₆ corresponding to the viscosity index.

An SUS computer 65 receives signal KV₂₁₀ and provides a signal E₇ corresponding to the term SUS in accordance with the received signals and equation 6 as hereinafter explained.

An SUS 210 computer 68 receives signal E₇ and applies signal E₈ corresponding to the term SUS₂₁₀ in accordance with the received signal and equation 7 as hereinafter explained.

A VI_(DWC).sbsb.O computer 70 receives signal KV₂₁₀, API, S and E₆ and provides a signal E₉ corresponding to the term VI_(DWC).sbsb.O in accordance with the received signals and and equation 8 as hereinafter explained.

A VI_(DWC).sbsb.P computer 72 receives signal E₈ and E₉ and provides a signal E₁₀ corresponding to the term VI_(DWC).sbsb.P in accordance with the received signals and equation 9. Subtracting means 76 performs the function of equation 10 by subtracting signal E₁₀ from a direct current voltage V₉ corresponding to the term VI_(RP), in equation 10, to provide a signal E₁₁ corresponding to the term ΔVI in equation 10.

A ΔRI computer 79 receives signals API, KV₂₁₀, S, E₆ and E₁₁ and provides a signal ΔRI in accordance with the received signals and equation 11, as hereinafter explained.

A J computer 80 receives signals T, ΔRI, KV₂₁₀, S, E₆ and E₁₁ and provides a signal E₁₃ corresponding to the term J in accordance with the received signals and equation 12 as hereinafter explained to a divider 83.

Signal SOLV is provided to a multiplier 82 where it is multiplied by a direct current voltage V₂ corresponding to a value of 100 to provide a signal corresponding to the term (SOLV)(100) in equation 13. The product signal is applied to divider 83 where it is divided by signal E₁₃ to provide signal C corresponding to the desired new charge oil flow rate.

It would be obvious to one skilled in the art that if the charge oil flow rate was maintained constant and the MP flow rate varied, equation 13 would be rewritten as

    SO=(J)(CHG)/100                                            (14)

where SO is the new MP flow rate. Control means 40 would be modified accordingly.

Referring now to FIG. 3, H computer 50 includes summing means 112 receiving signal KV₂₁₀ and summing it with a direct current voltage C₁ to provide a signal corresponding to the term [KV₂₁₀ +C₁ ] shown in equation 1. The signal from summing means 112 is applied to a natural logarithm function generator 113 which provides a signal corresponding to the natural log function generator 113A which in turn provides signal E₁.

Referring now to FIG. 4, K signal means 55 includes summing means 114 summing direct current voltage T₁₅₀ and C₃ to provide a signal corresponding to the term [T₁₅₀ +C₃ ] which is provided to a natural log function generator 113B which in turn provides a signal corresponding to the natural log of the sum signal from summing means 114. Subtracting means 115 subtracts the signal provided by function generator 113B from a direct current voltage C₂ to provide a signal corresponding to the numerator of equation 3. A divider 116 divides the signal from subtracting means 115 with a direct current voltage C₄ to provide signal E₃.

Referring now to FIG. 5, H signal means 53 includes subtracting means 117 which subtracts signal E₁ from signal E₂ to provide a signal corresponding to the term H₁₅₀ -H₂₁₀, in equation 4, to a divider 118. Divider 118 divides the signal from subtracting means 117 by signal E₃. Divider 118 provides a signal which is summed with signal E₁ by summing means 119 to provide signal E₄ corresponding to H₁₀₀.

Referring now to FIG. 6, a direct current voltage V₃ is applied to a logarithmic amplifier 120 in KV computer 60. Direct current voltage V₃ corresponds to the mathematical constant e. The output from amplifier 120 is applied to a multiplier 122 where it is multiplied with signal E₄. The product signal from multiplier 122 is applied to an antilog circuit 125 which provides a signal corresponding to the term exp (H₁₀₀) in equation 5. The signal from circuit 125 is multiplied with the output from logarithmic amplifier 120 by a multiplier 127 which provides a signal to antilog circuit 125A. Circuit 125A is provided to subtracting means 128 which subtracts a direct current voltage C₁ from the signal from circuit 125A to provide signal E₅.

Referring now to FIG. 7, VI signal means 63 is essentially memory means which is addressed by signals E₅, corresponding to KV₁₀₀, and signal KV₂₁₀. In this regard, a comparator 130 and comparator 130A represent a plurality of comparators which receive signal E₅ and compare signal E₅ to reference voltages, represented by voltages R₁ and R₂, so as to decode signal E₅. Similarly, comparators 130B and 130C represent a plurality of comparators receiving signal KV₂₁₀ which compare signal KV₂₁₀ with reference voltages RA and RB so as to decode signal KV₂₁₀. The outputs from comparators 130 and 130B are applied to an AND gate 133 whose output controls a switch 135. Thus, should comparators 130 and 130B provide a high output, AND gate 133 is enabled and causes switch 135 to be rendered conductive to pass a direct current voltage V_(A) corresponding to a predetermined value, as signal E₆ which corresponds to VI. Similarly, the outputs of comparators 130 and 130C control an AND gate 133A which in turn controls a switch 135A to pass or to block a direct current voltage V_(B). Similarly, another AND gate 133B is controlled by the outputs from comparators 130A and 130B to control a switch 135B so as to pass or block a direct current voltage V_(C). Again, an AND gate 133C is controlled by the outputs from comparators 130A and 130C to control a switch 135C to pass or to block a direct current voltage V_(D). The outputs of switches 135 through 135C are tied together so as to provide a common output.

Referring now to FIG. 8, the SUS computer 65 includes multipliers 136, 137 and 138 multiplying signal KV₂₁₀ with direct current voltages C₉, C₇ and C₅, respectively, to provide signals corresponding to the terms C₉ (KV₂₁₀), C₇ (KV₂₁₀) and C₅ (KV₂₁₀), respectively in equation 6. A multiplier 139 effectively squares signal KV₂₁₀ to provide a signal to multipliers 140, 141. Multiplier 140 multiplies the signal from multiplier 139 with a direct current voltage C₁₀ to provide a signal corresponding to the term C₁₀ (KV₂₁₀)² in equation 6. Multiplier 141 multiplies the signal from multiplier 139 with signal KV₂₁₀ to provide a signal corresponding to (KV₂₁₀)³. A multiplier 142 multiplies the signal from multiplier 141 with a direct current voltage C₁₁ to provide a signal corresponding to the term C₁₁ (KV₂₁₀)³ in equation 6. Summing means 143 sums the signals from multipliers 136, 140 and 142 with a direct current voltage C₈ to provide a signal to a multiplier 144 where it is multiplied with a direct current voltage C₁₂. The signal from multiplier 137 is summed with a direct current voltage C₆ by summing means 145 to provide a signal corresponding to the term [C₆ +C₇ (KV₂₁₀ ]. A divider 146 divides the signal provided by summing means 145 with the signal provided by multiplier 144 to provide a signal which is summed with the signal from multiplier 138 by summing means 147 to provide signal E₇.

Referring now to FIG. 9, SUS₂₁₀ computer 68 includes subtracting means 148 which subtracts a direct current voltage C₁₆ from another direct current voltage C₁₅ to provide a signal corresponding to the term (C₁₅ -C₁₆) in equation 7. The signal from subtracting means 148 is multiplied with a direct current voltage C₁₄ by a multiplier 149 to provide a product signal which is summed with another direct current voltage C₁₃ by summing means 150. Summing means 150 provides a signal corresponding to the term [C₁₃ +C₁₄ (C₁₅ -C₁₆)] in equation 7. The signal from summing means 150 is multiplied with signal E₇ by a multiplier 152 to provide signal E₈.

Referring now to FIG. 10, VI_(DWC).sbsb.O computer 70 includes multipliers 160, 161 and 162 which effectively square signals S, E₆ and KV₂₁₀, respectively, and provide corresponding signals. Multipliers 165, 166 multiply signal S with a direct current voltage C₁ and signal E₆, respectively, to provide product signals. Multipliers 169, 170 multiply signal KV₂₁₀ with signals E₆ and API, respectively, to provide product signals. Multipliers 175 through 180 multiply the signals from multipliers 160, 166, 161, 169, 162 and 170, respectively, with direct current voltages C₂₁, C₂₄, C₂₀, C₂₃, C₁₉ and C₂₂, respectively, to signals corresponding to the terms C₂₁ (S)², C₂₄ (VI)(S), C₂₀ (VI)², C₂₃ (KV₂₁₀)(VI), C₁₉ (KV₂₁₀)² and C₂₂ (API)(KV₂₁₀), respectively, in equation 8. Summing means 182 sums the signals from multipliers 175, 176, 177, 179 and 180, to effectively sum the positive terms of equation 8, and provides a corresponding sum signal. The negative terms of equation 8 are effectively summed when summing means 185 sums the signals from multipliers 165, 178 with a direct current voltage C₁₇. Subtracting means 187 subtracts the signal provided by summing means 185 from the signal provided by summing means 182 to provide signal E₉.

VI_(DWC).sbsb.P computer 72 shown in FIG. 11, includes a natural logarithm function generator 190 receiving signal E₈ and providing a signal corresponding to the term 1nSUS₂₁₀ to multipliers 192 and 194. Multiplier 192 multiplies the signal from function generator 190 with a direct current voltage C₂₆ to provide a signal corresponding to the term C₂₆ 1nSUS₂₁₀ in equation 9. Multiplier 194 effectively squares the signal from function generator 190 to provide a signal that is multiplied with the direct current voltage C₂₇ by a multiplier 196. Multiplier 196 provides a signal corresponding to the term C₂₇ (1nSUS₂₁₀) in equation 9. Subtracting means 198 subtracts the signals provided by multiplier 192 from the signal provided by multiplier 196. Summing means 200 sums the signal from subtracting means 198 with a direct current voltage C₂₅. A multiplier 202 multiplies the sum signal from summing means 200 with a direct current voltage POUR to provide a signal which is summed with signal E₉ by summing means 204 which provides signal E₁₀.

Referring to FIG. 12, multiplier 220 in ΔRI computer 79 effectively squares signal API while multipliers 222 and 224 multiply signal E₁₁ with signals API and KV₂₁₀, respectively, to provide product signals. Multipliers 226, 228 and 230 multiply signal KV₂₁₀ with signal API, a direct current voltage C₂₉ and signal E₆, respectively. Multipliers 235, 238 effectively square signals E₆ and S to provide product signals. Multiplier 239 multiplies signal E₆ with signal S. Multipliers 241 through 248 multiply the product signals from multipliers 220, 222, 224, 226, 230, 235, 238 and 239, respectively, with direct current voltages C₃₂, C₃₁, C₃₇, C₃₃, C₃₅, C₃₄, C₃₀ and C₃₆, respectively, to provide signals corresponding to the terms C₃₂ (API)², C₃₁ (ΔVI)(API), C₃₇ (ΔVI)(KV₂₁₀), C₃₃ (API)(KV₂₁₀), C₃₅ (VI)(KV₂₁₀), C₃₄ (VI)², C₃₀ (S)² and C₃₆ (VI)(S), respectively, in equation 11. Summing means 250 effectively sums the positive terms of equation 11 when it sums a direct current voltage C₂₈ with the signals from multipliers 228, 242, 243, 244, 246 and 248 to provide a sum signal. Summing means 252 effectively sums the negative terms of equation 11 when it sums the signals from multipliers 241, 245 and 247 to provide a sum signal. Subtracting 255 subtracts the sum signal provided by summing means 252 from the sum signal provided by summing means 250 to provide a signal which is multiplied with a direct current voltage C₃₈ by a multiplier 256. Multiplier 256 provides signal ΔRI.

Referring now to FIG. 13, J computer 80 includes multipliers 260, 261 multiplying signal ΔRI with a direct current voltage C₄₀ and signal E₁₁, respectively, to provide product signals. Multipliers 264, 265 multiply signal T with signals E₁₁ and KV₂₁₀, respectively, to provide product signals. Multipliers 269, 270 multiply signals E₆ and S, respectively, with direct current voltage C₄₃ and C₄₄, respectively, to provide signals corresponding to the terms C₄₃ (VI) and C₄₄ (S), respectively, in equation 12. A multiplier 273 effectively squares signal S to provide a signal which is multiplied with a direct current voltage C₄₁ by a multiplier 274 to develop a signal corresponding to the term C₄₁ (S)² in equation 12. Multipliers 276, 277 and 278 multiply the signals from multipliers 261, 264 and 265, respectively, with direct current voltages C₄₅, C₄₆ and C₄₂, respectively, to provide signals corresponding to the terms C₄₅ (ΔRI)(ΔVI), C₄₆ (ΔVI)(T) and C₄₂ (KV₂₁₀)(T), respectively.

Summing means 280 sums the positive terms of equation 12 when it sums the signals from multipliers 260, 269, 274 and 276 to provide a corresponding signal. Summing means 284 effectively sums the negative terms of equation 12 when it sums a direct current voltage C₃₄ with the signals from multipliers 270, 277 and 278 to provide a sum signal. Subtracting means 288 subtracts the signal provided by summing means 284 from the signal provided by subtracting means 280 to provide signal E₁₃.

The present invention as hereinbefore described controls an MP refining unit receiving light sweet charge oil to achieve a desired charge oil flow rate for a constant MP flow rate. It is also within the scope of the present invention, as hereinbefore described, to control the MP flow rate while the light sweet charge oil flow is maintained at a constant rate. 

What is claimed is:
 1. A control system for a refining unit having an extractor receiving light sweet charge oil and N-methyl-2-pyrrolidone solvent and providing raffinate and extract-mix which are subsequently processed to recover the N-methyl-2-pyrrolidone and to yield refined oil and extract oil, respectively, comprising gravity analyzer means for analyzing the light sweet charge oil and providing a signal API corresponding to the API gravity of the light sweet charge oil, sulfur analyzer means for analyzing the light sweet charge oil and providing a signal S corresponding to the sulfur content of the light sweet charge oil, viscosity analyzer means for analyzing the light sweet charge oil and providing signals KV₁₅₀ and KV₂₁₀ corresponding to the kinematic viscosities of the light sweet charge oil corrected to 150° F. and 210° F., respectively, flow rate sensing means for sensing the flow rates of the light sweet charge oil and the methyl-2-pyrrolidone and providing signals CHG and SOLV corresponding to the sensed flow rates of the light sweet charge oil and the N-methyl-2-pyrrolidone, respectively, temperature sensing means for sensing the temperature of the extract mix and providing a signal T corresponding thereto, and control means connected to all the analyzer means, to flow rate sensing means and to the temperature sensing means for controlling one of the flow rates of the light sweet charge oil and the N-methyl-2-pyrrolidone flow rates while maintaining the other flow rate constant in accordance with signals API, S, KV₁₅₀, CHG, SOLV and T.
 2. A system as described in claim 1, in which the control means includes VI signal means connected to the viscosity analyzer means for providing a signal VI corresponding to the viscosity index of the light sweet charge oil in accordance with kinematic viscosity signals KV₁₅₀ and KV₂₁₀ ; SUS₂₁₀ signal means connected to the viscosity analyzer means for providing a signal SUS₂₁₀ corresponding to the charge oil viscosity in Saybolt Universal Seconds corrected to 210° F.; ΔVI signal means connected to the viscosity analyzer means, to the gravity analyzer means, to the sulfur analyzer means, to the VI signal means and to the SUS₂₁₀ signal means and receiving a direct current voltage VI_(RP) corresponding to the viscosity index of the refined oil at the predetermined temperature for providing a signal ΔVI, corresponding to a change in viscosity index, in accordance with signals KV₂₁₀, API, S, VI and SUS₂₁₀ and voltage VI_(RP) ; ΔRI signal means connected to the viscosity analyzer means, to the sulfur analyzer means, to the ΔVI signal means, to the gravity analyzer means and to the VI signal means for providing a signal ΔRI corresponding to a change in refractive index between the charge oil and the raffinate in accordance with signals KV₂₁₀, S, ΔVI, API and VI; J signal means connected to the ΔVI signal means, to the ΔRI signal means, to the temperature sensing means, to the sulfur analyzer means, to the viscosity analyzer means and to the VI signal means for providing a J signal corresponding to an N-methyl-2-pyrrolidone dosage for light sweet charge oil in accordance with the signals ΔRI, ΔVI, T, KV₂₁₀, VI and S; control signal means connected to the J signal means and to the flow rate sensing means for providing a control signal in accordance with the J signal and one of the sensed flow rate signals; and apparatus means connected to the control signal means for controlling the one flow rate of the light sweet charge oil and N-methyl-2-pyrrolidone flow rates in accordance with the control signal.
 3. A system as described in claim 2 in which the SUS₂₁₀ signal means includes SUS signal means connected to the viscosity analyzer means, and receiving direct current voltages C₅ through C₁₂ for providing a signal SUS corresponding to an interim factor SUS in accordance with signal KV₂₁₀, voltages C₅ through C₁₂ and the following equation: ##EQU1## where C₅ through C₁₂ are constants; and SUS₂₁₀ network means connected to the SUS signal means and to the signal means and receiving direct current voltages C₁₃ through C₁₆ for providing signal SUS₂₁₀ to the ΔVI signal means in accordance with signal SUS, voltages C₁₃ through C₁₆ and the following equation:

    SUS.sub.210 =[C.sub.13 +C.sub.14 (C.sub.15 -C.sub.16)]SUS,

where C₁₃ through C₁₆ are constants.
 4. A system as described in claim 3 in which the VI signal means includes K signal means receiving direct current voltages C₂, C₃, C₄ and T₁₅₀ for providing a signal K₁₅₀ corresponding to a constant in accordance with voltages C₂, C₃, C₄ and T₁₅₀, and the following equation:

    K.sub.150 =[C.sub.2 -1n(T.sub.150 +C.sub.3)]/C.sub.4,

where C₂ through C₄ are constants, and T₁₅₀ corresponds to a temperature of 150° F.; H₁₅₀ signal means connected to the viscosity analyzer means and receiving a direct current voltage C₁ for providing a signal H₁₅₀ corresponding to a viscosity H value for 150° F. in accordance with signal KV₁₅₀ and voltage C₁ in the following equation:

    H.sub.150 =1n1n(KV.sub.150 +C.sub.1),

where C₁ is a constant; H₂₁₀ signal means connected to the viscosity analyzer means and receiving voltage C₁ for providing a signal H₂₁₀ corresponding to a viscosity H value for 210° F. in accordance with signal KV₂₁₀, voltage C₁ and the following equation:

    H.sub.210 =1n1n(KV.sub.210 +C.sub.1),

H₁₀₀ signal means connected to the K signal means, to the H₁₅₀ signal means and the H₂₁₀ signal means for providing a signal H₁₀₀ corresponding to a viscosity H value for 100° F. in accordance with signals H₁₅₀, H₂₁₀ and K₁₅₀ and the following equation:

    H.sub.100 =H.sub.210 +(H.sub.150 -H.sub.210)/K.sub.150,

KV₁₀₀ signal means connected to the H₁₀₀ signal means and receiving voltage C₁ for providing a signal KV₁₀₀ corresponding to a kinematic viscosity for the charge oil corrected to 100° F. in accordance with signal H₁₀₀, voltage C₁, and the following equation:

    KV.sub.100 =exp[exp][(H.sub.100)]-C.sub.1,

and VI memory means connected to the KV₁₀₀ signal means and to the viscosity analyzer means having a plurality of signals stored therein, corresponding to different viscosity indexes and controlled by signals KV₁₀₀ and KV₂₁₀ to select a stored signal and providing the selected stored signal as signal VI.
 5. A system as described in claim 4 in which the ΔRI signal means receives direct current voltages corresponding to constants C₂₈ through C₃₈ and provides signal ΔRI in accordance with signals KV₂₁₀, S, ΔVI, API and VI, the received voltages and the following equation:

    ______________________________________                                         ΔRI =                                                                            [C.sub.28 + C.sub.29 (KV.sub.210) - C.sub.30 (S).sup.2 + C.sub.31              (ΔVI)                                                                     (API) - C.sub.32 (API).sup.2                                                  +C.sub.33 (API)(KV.sub.210) + C.sub.34 (VI).sup.2 - C.sub.35                   (KV.sub.210)                                                                    (VI) + C.sub.36 (VI)(S)                                                       +C.sub.37 (ΔVI)(KV.sub.210)]C.sub.38.                            ______________________________________                                    


6. A system as described in claim 5 in which the ΔVI signal means includes VI_(DWC).sbsb.O signal means connected to the sulfur analyzer means, to the viscosity analyzer means, to the gravity analyzer means and to the VI signal means, and receiving direct current voltages C₁₇ through C₂₀ for providing a first signal VI_(DWC).sbsb.O corresponding to the viscosity index of the dewaxed charge oil having a pour point of 0° F., in accordance with signals S, VI, KV₂₁₀, and API, voltages C₁₇ through C₂₄, and the following equation:

    ______________________________________                                         VI.sub.DWC.sbsb.O =                                                                    -C.sub.17 - C.sub.18 (S) + C.sub.19 (KV.sub.210).sup.2 + C.sub.20              (VI).sup.2 +                                                                    C.sub.21 (S).sup.2 +  C.sub.22 (API)(KV.sub.210)                              -C.sub.23 (KV.sub.210)(VI) + C.sub.34 (VI)(S),                         ______________________________________                                    

where C₁₇ through C₂₄ are constants; VI_(DWC).sbsb.P signal means connected to the VI_(DWC).sbsb.O signal means and to the SUS₂₁₀ signal means, and receiving direct current voltages C₂₅ through C₂₇ and Pour, providing a signal VI_(DWC).sbsb.P corresponding to the viscosity index of the dewaxed charge oil at the predetermined temperature, in accordance with signals VI_(DWC).sbsb.O and SUS₂₁₀, voltages C₂₅ through C₂₇ and Pour, and the following equation:

    ______________________________________                                         VI.sub.DWC.sbsb.P =                                                                    VI.sub.DWC.sbsb.O + (Pour)[C.sub.25 - C.sub.20 lnSUS.sub.210 +                  C.sub.27 (lnSUS.sub.210).sup.2 ],                                     ______________________________________                                    

where Pour is the pour point of the dewaxed product and C₂₅ through C₂₇ are constants; subtracting means connected to the first VI_(DWC).sbsb.P means and to the J signal means and receiving voltage VI_(RP) for subtracting voltage VI_(RP) from signal VI_(DWC).sbsb.O to provide the ΔVI signal to the J signal means.
 7. A system as described in claim 6 in which the flow rate of the light sweet charge oil is controlled and the flow of the MP is maintained at a constant rate and the control signal means receives signal SOLV from the flow rate sensing means, the J signal from the J signal means and a direct current voltage corresponding to a value of 100 and provides a signal C to the apparatus means corresponding to a new light sweet charge oil flow rate in accordance with the J signal, signal SOLV and the received voltage and the following equation:

    C=(SOLV)(100)/J,

so as to cause the flow of the light sweet charge oil to change to the new flow rate.
 8. A system as described in claim 6 in which the controlled flow rate is the N-methyl-2-pyrrolidone flow rate and the flow of the light sweet charge oil is maintained constant, and the control signal means is connected to the sensing means, to the J signal means and receives a direct current voltage corresponding to the value of 100 for providing a signal SO to the apparatus means corresponding to a new furfural flow rate in accordance with signals CHG and the J signal and the received voltage, and the following equation:

    SO=(J)(CHG)/100,

so as to cause the N-methyl-2-pyrrolidone flow to change to the new flow rate.
 9. A system as described in claim 7 or claim 8 in which the J signal means receives direct current voltages corresponding to constants C₃₉ through C₄₆ and provides the J signal in accordance with the received voltages, signals ΔRI, S, T, KV₂₁₀, VI and ΔVI, and the following equation:

    ______________________________________                                         J =  -C.sub.39 + C.sub.40 (ΔRI) + C.sub.41 (S).sup.2 - C.sub.42               (KV.sub.210) (T)                                                               +C.sub.43 (VI) - C.sub.44 (S) + C.sub.45 (ΔRI) (ΔVI) -             C.sub.46 (ΔVI) (T).                                                 ______________________________________                                     